Possibly they're are used to a particular sound from birth together with the fact that this sound is not 'life threatening'? You try making the slightest noise that causes vibration to the ground on the rivers that I fish and your fishing is fecked for a few hours - they may return after a period of 10/20 minutes, or so, but they won't feed for quite a long time after (especially chub)....
I am not disputing what you say barbelboi simply because I don't know what fecks your swim up but how do you know that it is the vibration or some noise etc that does it....could it possibly be something else like the fish are not feeding for example...which happens very often on rivers like the Trent.
Don't get me wrong.....I DO believe it is wise to err on the side of caution..to use a well known phrase....and keep noise to a minimum but I am not sure just how detrimental it is towards fishing....and I don't see how anybody else can be either.
I remember a bloke I used to fish with many, many years ago when we used to fish in places like Pophams Eau, Three Holes, Boston Drains and the like and he was dead against the notion that noise on the bank affected fishing....So much so.....he used to sing as loud as he could and stamp his feet on the ground whilst casting and striking and hooking fish at the same time and he did this for 5 minutes or so and he regularly caught 3 or 4 fish during this "performance"......just to prove his point.
The fish we are talking about was usually roach and skimmer bream......but the noise certainly never bothered them....why would it bother other species.
I also remember a miserable, drizzly day at Topcliffe long before it belonged to a syndicate and I banged my 3 or 4 banksticks in as usual and cast towards my favourite spot just under the willow trees on the opposite side. Unfortunately, my feeder dropped very short and dropped into the river only about a third of the way across which was less than 10 meters away from the bank. I was just about to retrieve my feeder when I had a bite...only minutes, seconds maybe after I had cast in, missed it but decided to keep the feeder there and I had an absolutely brilliant day...something like 12 barbel and several chub....had my PB barbel at the time during that session too 10.75lb. I had an audience of about 5 blokes sat on the bank above me watching me in the end.
Just an example where noise and vibration didn't make one iota of difference.
Maverick